1. Field of the Invention
The present invention relates generally to a scrambling system for audio frequency signals and more particularly is directed to a scrambling system for audio frequency signals suitable for use with a pay television broadcast system.
2. Description of the Prior Art
In a radio communication and a magnetic recording system, there is sometimes employed a scrambling system for audio frequency signals. As an example of the former, a pay television broadcast system is considered. In the pay television broadcast system, a broadcast station (transmitter) and a user (receiver) conclude a contract having such a content that the user pays the broadcast station in compensation for enjoying a particular television broadcast program. In the radio communication, the receiver is not limited in principle so that the scrambling system for audio frequency signals is employed to thereby enable only the user who concludes the contract with the broadcast station to enjoy the particular television broadcast. Meanwhile, as an example of the latter, a so-called automatic answering telephone is considered. When it is necessary to keep the secret of the recorded content of the automatic answering telephone, such secret information is recorded by employing the scrambling system and after that, the content of the information can be reproduced and known to a particular person by use of a predetermined decoder.
Roughly classified, there are proposed two scrambling systems. One system is that the audio signal data is re-arranged on its frequency axis, while the other system is that the audio signal data is re-arranged on its timebase. The present invention concerns the latter system. As the latter system, there are proposed the following systems: the polarity of the sampled value of an audio signal is changed in accordance with a predetermined rule; the audio signal is divided into frames on the timebase and then the order of the sampled values is changed within one frame; and the several frames thus divided on the timebase are changed in order. By the way, in the system in which the audio signal data is re-arranged on the timebase, except the last system mentioned above, the audio signal after being re-arranged in order becomes wide in frequency band as compared with the original audio signal so that if this audio signal re-arranged is transmitted through the communication or transmission path the band region of which is restricted, a distortion occurs in the audio signal upon re-arranging or decoding. The last system has less defects mentioned above and is particularly suitable as the scrambling system. In this case, however, the order of several frames is changed so that the audio signal is caused to change abruptly at the connected portion between the ends of the frames, thus mixing a noise into the audio signal upon decoding.
An audio signal of a sine wave as, for example, shown in FIG. 1 is considered. In this case, the audio signal is divided into blocks Bi on the timebase. Each of the blocks Bi is formed of four frames f.sub.1, f.sub.2, f.sub.3 and f.sub.4. Then, in each block Bi, the frames f.sub.1, f.sub.2, f.sub.3 and f.sub.4 are arranged in the sequential order of FIG. 1B, namely, in the sequential order of the frames f.sub.4, f.sub.3, f.sub.2 and f.sub.1. As will be clear from FIG. 1B, the audio signal thus obtained abruptly rises up or falls down at the boundary between the frames. Accordingly, if this audio signal is transmitted through the transmission path having the narrow transmission band region and particularly when the transmission path does not allow the high frequency component to pass therethrough, the signal waveform is blunted. Thus, when the audio signal is again re-arranged or decoded on the receiving side, the original audio signal is distorted or a noise is superimposed upon the original audio signal, etc.so that the quality of the audio signal is deteriorated.